Throughout the Midwest, potassium (K) nutrition is receiving more and more attention. Many crop advisers and scientists have noted K deficiency symptoms in areas where none previously existed. Also, unexpectedly large variance in soil test K levels has been observed from year to year. Concern over K nutrition has led to a re-examination of factors important to K soil fertility evaluation and control. This paper focuses on three primary areas important to Wisconsin agriculture: 1) historical trends in crop nutrient removal, 2) factors that influence soil test K, and 3) variability in crop response to applied K.